Ultrasonic scanning systems which use pulse-echo ultrasound to provide information regarding the position, configuration, composition, and other internal features of human body organs are widely used in medicine. There are some anatomical situations in which improved ultrasonic images can be obtained by scanning from within the bodily cavities and vessels. Intracavitary ultrasonic scanners have been extensively described in the literature, e.g., in U.S. Pat. No. 4,869,258 of W. Hetz. Common types of intracavitary scanners are available for insertion into a rectum, vagina, and alimentary tract. Intracavitary scanners permit close contact with the region of interest, allowing higher ultrasonic frequencies to be used thus providing better resolution than extracorporeal scanners. Intracavitary scanners can also avoid problems with overlying structures, especially bone and those containing gas. A device technologically related to intracavitary scanner is an intravascular ultrasonic scanner such as described in U.S. Pat. No. 5,000,185 of P. G. Yock used for examination of blood vessels.
Both intracavitary and intravascular scanners comprise a probe-like shaft used to place a scanning transducer, located at a tip of the shaft, at the desired location inside the bodily cavity or vessel. Both scanner categories are therefore often referred to as probe scanners, such nomenclature having been used, e.g., in U.S. Pat. No. 4,972,839 of B. A. Angelsen and in U.S. Pat. No. 4,930,515 of R. A. Terwilliger.
To aid in the following description, it is convenient to define a cylindrical coordinate system where the z axis coincides with the axis of the shaft at its tip, r is the radial direction from the shaft axis, and .THETA. is the rotational angle measured around the shaft axis. A voxel (volume element) in such a cylindrical coordinate system is identified by a parallelepiped of the size dr.sub.i, r.sub.i .multidot.d.THETA..sub.j, and dz.sub.k, centered at the r.sub.i, .THETA..sub.j, and z.sub.k coordinates. The scanned volume can then be divided into closely fitting voxels, each identified by its coordinate values of r.sub.i, .THETA..sub.j, and z.sub.k.
Many display modes are being used in probe scanners. The closest art to the present invention is a plan position indicator (PPI) mode: It is an r,.THETA. polar coordinate display where the radial distance r corresponds to radial ultrasound beam echo delay time, the rotational angle .THETA. corresponds to the beam rotation around the axis of the shaft, and intensity corresponds to a processed amplitude of an echo signal at each position of r and .THETA.. The PPI mode limitation to an r,.THETA. display plane represents a serious shortcoming. For example, a rectal scan of a prostate then produces only a single cross-sectional plane of the organ, which limits its diagnostic value. By manual insertion or retraction of the shaft, one can obtain the r,.THETA. display in planes corresponding to different values of z along the axis of the shaft, yet each presentation remains in an r,.THETA. plane.
There are several requirements for accurate registration in the three-dimensional cylindrical coordinate system r,.THETA.,z needed to establish an accurate correspondence between anatomical features and their scanned voxel value as stored in the computer memory voxel space: one requirement is that the transducer and the shaft be moved along a straight z axis identifiable with respect to the cavity; another requirement is that the transducer orientation be such that the beam direction is truly radial. Yet another requirement is that the axial (z) coordinate and the rotational (.THETA.) coordinate of the beam be accurately determined.
Current PPI-type probe scanner devices have no means to control the z-axis motion within the cavity. Further, in current scanner practice, measurement of the shaft position coordinates is typically made at the proximal base of the shaft. The shaft design is therefore a compromise: the shaft needs to be flexible enough to follow the possibly convoluted path required to reach the desired location in the bodily cavity, yet rigid enough to minimize the error in coordinates between the decoders at the proximal shaft base and the transducer at the distal shaft tip. When the shaft is relatively short, straight, and rigid, this technique works well. However using this method, it is difficult to design scanners for distant organs e.g., for the uterus where the shaft has to be long and flexible; in such cases, due to the dynamics of torque transmission, there is a variable delay in the rotational angle between the proximal and distal ends of the shaft, leading to an error in the measured value of the .THETA. scan coordinate. In order to bypass the error between the proximal angle measurement and the distal value of the .THETA. coordinate, U.S. Pat. No. 4,880,011 of S. Imade et al uses a decoder located at the distal catheter end. Imade's decoder adds significant bulk to the scanner.
An alternative to a mechanical rotation is an electronically steerable circular array of sensors, which by electronic switching and phase shifting accomplishes beam rotation. Such arrays for ultrasonic probe application, working in a circular PPI mode, have been described in the literature (e.g., see "A 100-Element Ultrasonic Circular Array For Endoscopic Application in Medicine and NDT" by H. P. Schwarz et al in the Annual International Conference of the IEEE Engineering and Biology Society, Vol. 12, No. 1:287-290, 1990). A steerable circular array still requires mechanical motion in the z direction.
In state-of-the-art ultrasonic probe scanners, voxel space storage and processing is not used, largely due to registration errors; the display of the probe scan is in real time, i.e., the information displayed is obtained from the scanner data with only minimal processing delay. A shortcoming of such presentation is that the repetitive stream of scan data is irretrievably two-dimensional, and cannot be used to improve the resolution or to reduce "speckle". "Speckle" is a random scintillation pattern on the display, an artifact unrelated to the anatomical structure, resulting from an interference between the waves backscattered from within the coherent ultrasonic resolution cell.